Java中unsafe操作实例总结

/**
* 比较obj的offset处内存位置中的值和期望的值，如果相同则更新。此更新是不可中断的。
* 
* @param obj 需要更新的对象
* @param offset obj中整型field的偏移量
* @param expect 希望field中存在的值
* @param update 如果期望值expect与field的当前值相同，设置filed的值为这个新值
* @return 如果field的值被更改返回true
*/
public native boolean compareAndSwapInt(Object obj, long offset, int expect, int update);

public final int getAndAdd(int delta) {  
  return unsafe.getAndAddInt(this, valueOffset, delta);
}

//unsafe.getAndAddInt
public final int getAndAddInt(Object var1, long var2, int var4) {
  int var5;
  do {
  /**获取原始值*/
    var5 = this.getIntVolatile(var1, var2);
  /**确认原始值没有被其它线程修改时，再执行更新var5+var4操作*/
  } while(!this.compareAndSwapInt(var1, var2, var5, var5 + var4));
  return var5;
}

/***
  * Sets the value of the integer field at the specified offset in the
  * supplied object to the given value. This is an ordered or lazy
  * version of <code>putIntVolatile(Object,long,int)</code>, which
  * doesn't guarantee the immediate visibility of the change to other
  * threads. It is only really useful where the integer field is
  * <code>volatile</code>, and is thus expected to change unexpectedly.
  *
  * @param obj the object containing the field to modify.
  * @param offset the offset of the integer field within <code>obj</code>.
  * @param value the new value of the field.
  * @see #putIntVolatile(Object,long,int)
  */
 public native void putOrderedInt(Object obj, long offset, int value);

 Segment<K,V> s0 =
    new Segment<K,V>(loadFactor, (int)(cap * loadFactor),
             (HashEntry<K,V>[])new HashEntry[cap]);
  Segment<K,V>[] ss = (Segment<K,V>[])new Segment[ssize];
  // 往数组下标为0的位置，写入s0: ss[0]=s0
  UNSAFE.putOrderedObject(ss, SBASE, s0); // ordered write of segments[0]

/***
  * Sets the value of the integer field at the specified offset in the
  * supplied object to the given value, with volatile store semantics.
  *
  * @param obj the object containing the field to modify.
  * @param offset the offset of the integer field within <code>obj</code>.
  * @param value the new value of the field.
  */
 public native void putIntVolatile(Object obj, long offset, int value);